Most vehicles in operation today are powered by internal combustion (IC) engines. Internal combustion engines typically have multiple cylinders or other working chambers where combustion occurs. The power generated by the engine depends on the amount of fuel and air that is delivered to each working chamber. The mass of air delivered into each working chamber per intake event is referred to as the mass air charge.
Air is typically delivered into the working chamber from an intake manifold. A throttle valve helps regulate the delivery of air from the outside environment into the intake manifold. Opening the throttle causes more air to enter the intake manifold, which tends to increase the manifold absolute pressure. Higher manifold absolute pressure causes more air to enter the working chamber which when combusted with fuel generates greater torque and power.
It is important to accurately estimate the mass air charge. Generally, fuel is delivered to the working chamber in proportion to the mass air charge estimate. If the mass air charge estimate is inaccurate, there may be improper combustion. This can result in poor performance and the generation of undesirable pollutants in the exhaust of the vehicle.
There are several ways to determine the mass air charge. One approach uses a mass air flow sensor. The mass air flow sensor, which is typically located in a line between the air cleaner and the throttle, measures the mass of air flowing into the intake manifold which is used to estimate the mass air charge. A drawback of using the air meter directly is that depending on how the estimate is done there can be either no direct relation or a time delay between the measured mass air and when air is inducted into a cylinder. This may cause the estimated cylinder air mass charge to differ from the actual value, especially during transient conditions.
Another approach is commonly referred to as a speed density system. In this approach, the mass air charge is calculated based on engine speed, inlet air temperature, and manifold absolute pressure (MAP) which is typically measured directly using a suitable sensor in the intake manifold.
There are a number of patent documents and other publications that discuss additional techniques for estimating mass air charge. For example, U.S. Pat. No. 6,760,656 (hereinafter referred to as the '656 patent) relates to a method for estimating cylinder air charge for a variable displacement engine that shifts between two modes of operation, one in which all the cylinders are fired and another in which half the available cylinders are fired. The cylinder air charge estimate is based on data provided by a manifold absolute pressure sensor, which directly measures manifold pressure and a throttle position sensor.